Fusion

Another type of nuclear reaction that can release huge amounts of energy is fusion. Fusion is a process in which the nuclei of two atoms combine to form a larger nucleus. As in fission, during fusion, a small fraction of the reactant mass is converted into energy.

On any day or night, you can detect the energy released by fusion reactions occurring far away from Earth. The sun and other stars are powered by the fusion of hydrogen into helium. Inside the sun, an estimated 600 million tons of hydrogen undergo fusion each second. About 4 million tons of this matter is converted into energy.

Matter within the sun and other stars exists as plasma. Plasma is a state of matter in which atoms have been stripped of their electrons. You can think of plasma as a gas containing two kinds of particles—nuclei and electrons. Although fusion occurs at millions of degrees Celsius, plasma can exist at much lower temperatures. Scientists estimate that more than 99 percent of matter in the universe is plasma.

Fusion may someday provide an efficient and clean source of electricity. Scientists envision fusion reactors fueled by two hydrogen isotopes, deuterium (hydrogen-2) and tritium (hydrogen-3). The fusion of deuterium and tritium produces helium, neutrons, and energy.

${}_1{}^2\mathrm{H}+{}_1{}^3\mathrm{H}\to {}_2{}^4\mathrm{He}+{}_0{}^1\mathrm{n}+\mathrm{energy}$

Scientists face two main problems in designing a fusion reactor. They need to achieve the high temperatures required to start the reaction, and they must contain the plasma.

Figure 21 The Tokamak Fusion Test Reactor at the Princeton Plasma Physics Laboratory in Princeton, New Jersey, was one of the very few fusion reactors that have been built. It was retired in 1997, after 15 years of experimentation.

Table of Contents